I want to change the locations of my Pictures, Videos and Music folders.

I found some instructions which involve a simple edit to
~.config/user-dirs.dirs

I make the changes, and sure enough the Pictures shortcut etc now go to
my new locations. The screensaver Pictures folder is the new one too.

However after a reboot, all the entries in user-dirs.dirs are reset to
"$HOME"

The changes I've tried to make are of the form
XDG_MUSIC_DIR="/media/Seagate/Music"

There's a comment at the top of user-dirs.dirs:

# This file is written by xdg-user-dirs-update
# If you want to change or add directories, just edit the line you're
# interested in. All local changes will be retained on the next run
# Format is XDG_xxx_DIR="$HOME/yyy", where yyy is a shell-escaped
# homedir-relative path, or XDG_xxx_DIR="/yyy", where /yyy is an
# absolute path. No other format is supported.
#

Which ambiguously suggests both that the file is written by software,
and that a change to it will persist.

Any suggestions as to what I need to do?